1. Field Of The Invention
The invention relates generally to magnetic recording. More particularly, the invention relates to tracking control.
2. Description Relative to the Prior Art
With reference to the magnetic recording art, tracking is the process of keeping a magnetic head, either record or playback, on a particular track. The purpose of tracking control is to adjust the position of the magnetic head relative to the track or vice versa. In a recording mode, tracking control serves to maximize useable recording area without overwrite of existing record tracks. In a playback mode, tracking control functions to maintain a playback head aligned with a record track for maximum signal-to-noise output.
Tracking control is particularly advantageous for magnetic recording apparatus in which the density of tracks is very high. When playback occurs on apparatus other than that which was used for recording, tracking control has been found to be even more advantageous because of mechanical tolerance build-up inherent in different apparatus, variations in magnetic medium, either tape or disk, and other secondary tolerances.
In positioning a magnetic head, commonly the head is locked into direct coincidence with a selected track by means of a closed-loop head-positioning servomechanism. The "servo" utilizes information prerecorded on the magnetic medium. It is known in the prior art to use one or more tracks (on a disk or tape) exclusively for recorded servo signals which are played back by a magnetic head dedicated exclusively to servo activities.
Commonly, servo information is recorded on opposite sides of a reference centerline. If centered directly over the reference centerline, the magnetic head reads two signals of equal amplitude; if off-center, the amplitude of one signal played back exceeds that of the other. The closed-loop head-positioning servo serves to drive the magnetic head in a direction so as to equally match the respective amplitudes of the two servo signals played back, thereby centering the head over the reference centerline.
Although they generally have been found to work well for their intended purpose, it is axiomatic that prior art tracking control systems introduce additional cost and complexity into magnetic recording apparatus. First, such systems require precision preformatting apparatus for recording servo signals on either side of a track centerline. In many cases the signals differ from each other not only in track location but also in content, such as frequency or the like. Second, playback electronics are required to compare the servo signals played back with respect to each other. In some cases, this can require multiple playback channels having matched electrical characteristics in order to make a meaningful comparison.